1. Technical Field
The present disclosure relates to a caption displaying method and associated embedded system, and more particularly to a caption display method applied to digital televisions and associated embedded system.
2. Description of Related Art
The rising demand for digital televisions due to the increasing popularity of digital programs has become a global trend. The European television standard, the digital video broadcasting (DVB), and the American television standard, the advanced television systems committee (ATSC), are the two mainstreams of current digital television standards. In the European standard, subtitle streams may be contained in image signals and captioned as an aid to ease the content comprehensibility of the program viewers.
The head-end system of digital televisions sends out or reads the subtitle streams contained in an image signal outputted from a digital video disc reader (DVD reader). The subtitle streams contain digital television captions which can be displayed on a screen by a caption decoding apparatus included in digital televisions. The decoding process described above may be realized utilizing software executables benefited from the more powerful microprocessors, graphics engines included in television chips in digital televisions.
The European television caption standards may utilize the color indices for defining the colors in caption, by utilizing fewer bits for representing caption colors in transmission. For example, 2 bits may be used to represent 4 colors, and 4 bits for representing 16 colors . . . etc, and therefore, each image that the caption image file sends out from the head-end system in a digital television is either represented in 2 bits or in 4 bits instead of using the original 16 bits or 32 bits. Thus, the head-end system is only required to send out the corresponding color index table of each caption image file to the digital television for displaying the desired colors utilizing the caption image file and the color index table.
Most digital television systems utilize the ARGB (Alpha, Red, Green, Blue) color mode for image displaying. The digital televisions require the color index tables for data format conversion to convert the color index data to ARGB data for displaying. The ARGB data size is much larger than the color index data. The conversion is often executed in the last image outputting stage for the digital televisions to reduce the memory demand.
The European digital television standard allows displaying plural captions according to plural speakers and the speaker's tones as an aid provided for the hearing impaired. Referring to FIG. 1 which illustrates a conventional system, head-end system 19 may send out two caption image files simultaneously, including a first caption image file 110 and a second caption image file 120, and the corresponding two color index tables, including a first color index table 111 and a second color index table 121, for the purpose of displaying a first caption image 11 and a second caption image 12 at different locations on the display screen simultaneously. Due to the discrepancies between the contents of the first color index table 111 and the second index table 121 sent out from the head-end system 19, for example, if the color index value “15” in the first and second color index tables corresponds to different ARGB data, then a false color may be displayed since an incorrect color index table was referred when a television 18 simultaneously display the first caption image 11 and the second caption image 12.